1. Field of the Invention
The invention relates to a charge and discharge control apparatus and charge and discharge control method for a secondary battery and, more particularly, to a charge and discharge control apparatus and charge and discharge control method for a secondary battery, which use feedforward control and feedback control in combination to execute charge and discharge control.
2. Description of the Related Art
When the state of charge (hereinafter, referred to as SOC) of a secondary battery falls outside an appropriate state and enters an overcharged state or an overdischarged state, performance degradation, or the like, may occur, so charging and discharging is controlled in order to maintain an appropriate SOC. In addition, as in the case of a lithium ion battery, some secondary batteries require strict limiting control over charging current or charging voltage in order to prevent precipitation, or the like, of lithium metal. In addition, in any secondary batteries, for example, in a low-temperature environment, a permissible electric power for charging and discharging is significantly limited in view of performance protection, or the like, so it is necessary to appropriately control charging and discharging electric power in accordance with a battery temperature.
One of charge and discharge control modes of a secondary battery is a control mode that uses feedforward control and feedback control in combination. Feedforward control is, for example, a control mode in which a permissible electric power value corresponding to a battery temperature is set as a target value and then a control signal that implements the target value is unilaterally given. On the other hand, feedback control is, for example, a control mode in which a control parameter, such as a charging voltage, is detected and then a control signal is given in order to reduce a difference between a target value of the control parameter and the detected value of the control parameter. Generally, according to feedforward control, it is possible to maintain a stable charging voltage, or the like; however, it is difficult to sufficiently perform control when an external influence (disturbance) acts on a charge/discharge voltage, or the like. Therefore, in combination with feedback control, for example, a charging voltage is managed so as not to exceed an upper limit voltage value (target value). That is, feedback control is performed while referring to a result, so a variation range of the control parameter increases; however, even when disturbance occurs and appears in a detected value, the detected value may be fed back to manage the target value.
There are proposed some control apparatuses that are intended to achieve high-efficiency charging and discharging in a low-temperature environment. For example, Japanese Patent Application Publication No. 10-108380 (JP-A-10-108380) describes a control apparatus that compares a chargeable depth of charge calculated from a detected battery temperature and a detected charging voltage with a prestored function that uses a battery temperature, a depth of charge and a charging voltage as parameters and that controls a charging electric power to an optimal charging electric power even in any environments (for example, a charging electric power is decreased in a low-temperature environment, and a charging electric power is increased at a normal temperature). In addition, Japanese Patent Application Publication No. 2000-92603 (JP-A-2000-92603) describes an output control apparatus that increases the output limiting value of a battery as the degree of decrease in battery voltage or SOC of the battery increases to thereby increase the degree of output limiting.
The control apparatuses described in JP-A-10408380 and JP-A-2000-92603 vary a charging and discharging electric power in accordance with a battery temperature, or the like, to thereby intend to efficiently perform charging and discharging in response to an environmental variation, or the like; however, those control apparatuses do not consider controllability of charge and discharge operation. That is, as described above, for the secondary battery in a low-temperature environment, a charging and discharging electric power is significantly limited in order to prevent performance degradation, or the like, of the battery; however, it is not easy to accurately control a significantly limited low charging and discharging electric power.
Particularly, in charging a lithium ion battery in a low-temperature environment, there is, for example, a problem that lithium metal easily precipitates on a negative electrode surface, and a charging electric power is further limited as compared with another secondary battery, so it is difficult to accurately control charging electric power. In addition, in an in-vehicle secondary battery that is charged as an engine is driven, when a charging electric power is significantly limited in a low-temperature environment, it is difficult to perform driving control over the engine, so controllability of charging operation deteriorates.